Picture load callback and size access

[mediavrog]

Please add this functionality, so it is easier to access the width/height (and therefore scaleRatio) of the underlying original image, which is useful for lots of View calculation and other filters.
Furthermore, I added an OnImageLoadedListener. A usecase would be to have a GPUImageTwoInputFilter, which should receive the rendered bitmap of another filter as the second texture (GPUImageTwoInputFilter#setBitmap(Bitmap bitmap))

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

Maik Vlcek seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}